High output reciprocating engines tend to have limitations caused by extremely high temperature. Consequently, resistance to thermal stress is a key parameter in piston design. Such loading causes excessive piston expansion, distortion, loss of strength, thermal fatigue, cracking and piston seizures. The high temperatures also cause oil decomposition and formation of varnish and coke, so that piston rings stick in their grooves and fail to seal properly against cylinder walls.
Various known designs are used to counter the effects of extremely high engine temperature. Some engines have arrangements to spray lubricating oil on the underside of the piston crowns to cool the pistons. In other engines, especially diesels requiring thicker pistons, oil passages are cored into the crowns. Such passages are quite effective because cooling oil circulates near the piston rings, where the need for heat reduction is greatest. Unfortunately, incorporating these passages into the crowns involves exotic foundry techniques and the manufacturing costs are high. Some pistons have their crowns separate from their skirts to facilitate formation of oil cooling passages. In fact, various two-piece piston designs are the norm for large pistons even though such designs carry penalties in terms of cost, weight and structural strength. Of course, weight and structural strength are especially critical in high performance engines.
FIG. 1 shows a typical prior art piston 10, inside of which is a wrist pin 12 defining a longitudinal chamber 14. Engine oil enters chamber 14 from a connecting rod (not shown) which has a bore through which the oil flows. Oil exits from chamber 14 through the wrist pin's passage 16 and enters passage 18 of the piston. From passage 18, oil goes to annular cavity 20 and through duct 22 into a central cavity 24. The oil accepts heat from the crown areas of piston 10 adjacent cavities 20 and 24. Oil drains from cavity 24 through egress 26. Also, since cavity 20 communicates to cavity 24, oil from cavity 20 ultimately drains through egress 26.
FIG. 2 shows another typical prior art piston 28 having a wrist pin 30 defining chamber 32. Oil exits chamber 32 through wrist pin passage 34, passes through piston passage 36 and then enters annular cavity 38. The oil accepts heat from the piston's crown 42 where annular cavity 38 is, and piston ring 40 is also cooled. Oil drains from cavity 38 through egresses 44, which open at locations of internal piston surface 46 not covered by journal boss 48.